Recovery of metal and sulfate values from electrochemical mining electrolytes

ABSTRACT

A method is described for recovering metal and sulfate values from electrolyte solutions which are withdrawn from the in-place electrolysis of sulfide ore bodies positioned beneath the earth&#39;&#39;s surface. The electrolysis of such ore bodies produces finely divided metals, metal ions and sulfate ions. The particular metal ions present in the electrolyte is dependent upon the type of sulfide ore body being electrolyzed and the conditions of the electrolysis. The present process provides a means for continuously or periodically withdrawing electrolyte and finely divided metals from the electrolysis, separating the metal values both as solids and as ions in solution, recovering the useful products therefrom, readjusting the electrolyte to the desired conditions of pH and ion concentration and returning the readjusted electrolyte to the electrolysis.

United States Patent 1191 Drinkard, Jr. et al.

636,321 11 1899 Craney 204/108 822,175 5/1906 Wolcott 204/105 R 10 C 1 Drawing Figure NITIIDITANN ELECTROLYTE SOLIDS F/NELV DIVIDED SEPARATION METAL COPPER FILTRATE OFT/0N- IIETAL REMOVWLQLQJ ACE 125. PURIFICATION ALKAL/ SULFATE LIME ORALKALI SULFATE PRECIPITATION ADDITION IRON OXIDE SOLIDS ff/$231M 0? HYDROXIDE SEPARATION ADM/0N ACID HALOGEN SALT ADDITION F'LTRA TE ADDITION READJUSTED ELECTROLYTE RECOVERY OF METAL AND SULFATE VALUES FROM ELECTROCHEMICAL MINING ELECTROLYTES Inventors: William F. Drinkard, Jr., Charlotte;

Henry S. Brown, Raleigh, both of NC.

Assignee: Mineral Research & Development Corporation, Charlotte, NC.

Filed: May 17, 1974 Appl. No.: 470,807

US. Cl 204/105 R; 204/107; 204/108;

423/144; 423/166 Int. Cl. C25C 1/00; C25C 1/12 Field of Search 204/107, 108, 112, 113,

References Cited UNITED STATES PATENTS 2,168,985 8/1939 Gulbrandson 423/36 3,586,498 6/1971 Kasey ..75/109 Primary Examiner-R. L. Andrews Attorney, Agent, or FirmHerbert M. Adrian, .lr.

[ 5 7 ABSTRACT A method is described for recovering metal and sulfate values from electrolyte solutions which are withdrawn from the in-place electrolysis of sulfide ore bodies positioned beneath the earths surface. The electrolysis of such ore bodies produces finely divided metals, metal ions and sulfate ions. The particular metal ions present in the electrolyte is dependent upon the type of sulfide ore body being electrolyzed and the conditions of the electrolysis. The present process provides a means for continuously or periodically withdrawing electrolyte and finely divided metals from the electrolysis, separating the metal values both as solids and as ions in solution, recovering the useful products therefrom, readjusting the electrolyte to the desired conditions of pH and ion concentration and l-loepfner 204/107 returning the readjusted electrolyte to the electrolysis.

US. Patent Oct. 21, 1975 ALKALI SULFATE ACID ADDITION WITHDRA MM ELECTROLYTE SOL/D5 F/NELYD/VIDED SEPARATION METAL F/LTRATE OPTION- METAL Q%3 PURIFICATION SULFATE L/ME ORALKAL/ PREEIPITAT/ON ADDITION AERA T/0M AND/0R SOL/D5 FURTHERALKAL/ SEPARATION Ammo HALOGEN SALT FILTRATE ADDITION READJUSTED E LE C TROLY TE RECOVERY OF METAL AND SULFATE VALUES FROM ELECTROCHEMICAL MINING ELECTROLYTES INTRODUCTION This invention relates to electrochemical mining of sulfide ores from earth located deposits, and more particularly to the method of recovering the metal and sulfate values from electrolyte solutions used in the electrolysis of such sulfide ores.

BACKGROUND OF THE INVENTION It has previously been suggested to directly mine metal values from sulfide ores. While theoretically the idea is feasible from an engineering standpoint, previous attempts to accomplish such electrochemical mining met with limited success. Initially, the reaction would proceed as theorized, but after only a relatively short application of electrical current to the ore body, the electrodes would polarize due to the buildup of an insulating shield of sulfur on the surface of the ore. Current efficiencies would rapidly deteriorate and the electrolysis would slow to an unacceptable rate. Continued application of electrical energy would soon result in total polarization of the electrodes and termination of the electrolysis.

Commonly assigned co-pencling applications Ser. No. 470,808 and Ser. No. 470,809, filed May 17, 1974 set forth methods whereby these difficulties and shortcomings have been overcome to thereby efficiently and continuously electrochemically mine metal values from sulfide ore deposits. Having solved the problem of continuous electrolysis, it has become apparent that it is desirable to recover all of the metal and sulfate values produced in the electrolysis of such sulfide ore deposits in a manner which minimizes both subsequent refining and purification of the metal values and provides for continuous operation.

Most valuable sulfide ore deposits would preferably be worked for their copper values, although the other valuable metals found in greater or lesser extents as the copper would also desirably be obtained and recovered. In particular, iron is often present in such sulfide ores along with copper in up to a major proportion of the metal value. It is often desirable to recover in the same process the iron in a form which is commercially salable. While the conditions of electrolysis can be controlled so as to yield primarily free metallic copper and those metals present with a lower electromotive force than copper, it is sometimes more feasible to carry out the electrolysis with higher decomposition voltages such that all of the metals are reduced to the free state. Under such conditions, metal separation procedures are generally required.

Under the conditions of electrolysis wherein only metals having a lower electromotive force than iron are reduced to the metallic state, iron ions accumulate and remain in the electrolyte solution thereby requiring continuous or periodic removal and readjustment of the electrolyte.

It is therefore an object of the present invention to provide a method for recovering metal and sulfate values from electrochemical mining electrolyte solutions.

It is another object of the present invention to provide a method for separating the metal and sulfate val ues from the electrolyte solution as valuable and commercially salable or usable products.

It is a further object of the present invention to provide a method for continuously or periodically withdrawing electrolyte solution from the electrolysis, separating the metal sulfate values therefrom, readjusting the electrolyte and returning the same to the electrolysis.

These and other objects of the present invention will become apparent to those skilled in the art from the description of the invention which follows:

THE INVENTION In accordance with the invention, a method is provided for recovering metal and sulfate values from electrolyte solutions withdrawn from the in situ electrolysis of sulfide ores, wherein said solutions are acidic and contain copper, iron, sulfate and halogen ions comprising separating the solids from said withdrawn electrolyte, adding an alkaline earth metal oxide or hydroxide to said electrolyte to increase the pH of said electrolyte to slightly acidic to about neutral to thereby form an insoluble precipitate with said sulfate ions, separating the precipitate from said electrolyte, precipitating iron from said electrolyte by further pH increases or aeration, separating said iron precipitate and readjusting said electrolyte by the addition of a strong inorganic acid and a halogen to an acidic pH and a halogen ion concentration of at least 1 molar.

As an optional step in the process, the electrolyte solution can be passed over iron, particularly scrap iron, prior to increasing the pH thereof to slightly acidic to about neutral to thereby plate out copper on the iron. Also, a choice of iron precipitate can be had by either increasing the pH to above about 10 to precipitate ferric and ferrous hydroxide or aerating to produce com mercial iron oxides.

It will be readily recognized that the present process eliminates the need to bring ore deposits to the earths surface and the additional processing steps often used in conventional mining, such as crushing, acid leaching, concentration of ore, and the like. Because the process can be controlled to bring only the useful products to the earths surfaces, waste disposal is substantially eliminated as well as contamination of surface waters in the washing or leaching of ores. Noxious fumes are not emitted, such as can be the case with conventional sulfide ore smelting wherein sulfur fumes are sometimes vented to the atmosphere.

DETAILS OF THE INVENTION The invention will be more fully understood by reference to the drawing which is a flow sheet setting forth the steps in the process of recovering metal and sulfate values from withdrawn electrolyte.

The electrolyte is withdrawn from in-place electrolysis of sulfide ore bodies in subterranean locations.

Briefly stated, the electrolysis utilizes a source of direct electrical current connected to electrodes positioned in subterranean ore deposits. The ore deposit itself is used as the anode with the cathode being placed in shafts or wells in the ore body. Electrolyte fills the shafts contacting the ore body and the cathode. The positive source of direct current is attached to the ore deposit wherever convenient such as by sinking a separate well or shaft into the ore body or, if the ore body is near the earths surface, by mere attachment there.

The negative terminal of the direct electrical current source is connected to a cathode which is positioned within a separate well or shaft penetrating into the ore body. The cathode containing shaft is filled with electrolyte which makes contact with the sulfide ore deposit. One or many such cathode containing shafts can be utilized with a single anode attachment.

The vertical well or shaft which contains the cathode, technically referred to as the catholyte compartment, is preferably sealed to enclose the electrolyte in the catholyte compartment in a position within the ore body. Electrolyte withdrawal means, including associated pipes and pumps, and electrolyte return means are connected to the catholyte compartment. Such withdrawal means provide for continuous or periodic withdrawing and replenishing of electrolyte solution.

In the operation of the electrolysis, a decomposition voltage is applied between the anode and the cathode, thus decomposing sulfide ore in contact with the electrolyte;. Metal values enter the electrolyte and, depending upon the decomposition voltage, metals are reduced at the cathode. Again, depending upon the decomposition conditions utilized, electrolyte concentration will change as the electrolysis proceeds, and therefore means for withdrawing product and/or electrolyte are preferably utilized to maintain continuous operation at preferred electrolyte concentrations. The excess buildup of certain ions can result in the polarization of electrodes and/or precipitation of certain metal and cationic values which otherwise are desirably retained in solution for more convenient recovery.

Many sulfide ore deposits contain more than one metal which is desirably recovered. Therefore, in the process of the present invention, methods are described wherein more than one metal may be isolated from the electrolyte. For instance, several widely found sulfide ore deposits contain both copper and iron in varying proportions. Additionally, smaller amounts of other metals may also be present. Electrolysis conditions can be controlled to yield the metal values as ions in solution, one or both metals as finely divided free metal, or mixtures of free metals, with additional metal ions being present in'the electrolyte solution.

The sulfide ores upon which the present invention are most applicable are those electrically conductive sulfide ores containing metals of groups IB, [[8, lVA, VA, VlB and VIII of the Periodic Chart of Elements as shown in Langes Handbook of Chemistry, Eighth Edition, pages 56 and 57. In particular, metals most frequently found in sulfide ores such as copper, nickel iron, lead, palladium, silver, cobalt and cadmium are recoverd by the present process.

Because the ores electrolyzed are sulfide ores, the electrolysis produces an increase in concentration of sulfate ions in the electrolyte. Under certain conditions, some particulated free sulfur is also formed. These sulfur and sulfate values are preferably removed from the electrolyte to maintain the most preferred electrolyte conditions and to recover the sulfate values as commercially usable and salable by-products.

Therefore, referring more particularly to the drawing, withdrawn electrolyte from the catholyte compartment is passed through solids separation means to remove finely divided metal. in the electrolysis, it is preferred to utilize a nonadhering cathode, such as titanium, which results in the formation of finely divided metal which settles to the bottom of thecatholyte compartment. The separated finely divided metal can be in either a relatively pure form, depending upon the conditions of electrolysis, or in admixture with other metals, including iron. Thus, optionally, metal purification and further refining or separation, can be applied to the separated metal.

The solids separation can conveniently take the form of conventional separation techniques including filtration, decanting, floatation, centrifugation, magnetic separation and the like. The resulting filtrate is then ready for further treatment.

The filtrate can optionally be passed over iron, particularly scrap iron, to deposit and remove copper therefrom. This optional step is dependent upon the copper ion concentration in the solution and the desirability of avoiding copper contamination in subsequent separation steps.

The sulfate ions are next removed from the electrolyte solution. Such removal is preferably effected by the addition of an alkaline earth metal oxide such as barium or calcium to thereby form an insoluble precipitate with said sulfate ions. As a practical matter, calcium oxide or lime is the most readily available and least expensive basic material to use. However, it is recognized that wherein the demand for a particular sulfate warrants the addition of another cation, the corresponding cation which forms the desired precipitate with the sulfate is added. When lime is added, the sulfate precipitates as calcium sulfate.

The filtrate of such sulfate precipitation is then treated to remove iron. The iron is removed by either aeration or by the addition of more hydroxyl ions. The hydroxyl ions are preferably added in the form of alkali metal hydroxides, such as sodium, potassium, lithium or ammonium hydroxide in an amount sufficient to increase the pH to above about 10. Again, the particular hydroxide added is dependent upon the availability and cost thereof. For instance, if there is a demand for the ammonium salts, then ammonium hydroxide would be added. However, under normal conditions, the most available hydroxide is sodium hydroxide and its addition is often preferred, particularly because it does not result in the addition of a further cation in the electrolyte solution which is different from that added in the electrolyte adjustment step. In the electrolyte adjustment step, it is preferred to add an alkali metal salt of halogen. By utilizing the same alkali metal in both instances, additional cation concentrations need not be contended with in the electrolyte which may require further or additional separation steps.

The hydroxide addition is made in an amount to increase the pH of the electrolyte to above about 10, and more preferably at the pH which best results in the precipitation of ferrous and ferric hydroxide or hydrated iron oxides under the conditions of residence time and temperatures. Consideration of the form of precipitate is also desirable, and such form is selected so as to provide a more readily filtrable solution.

Where economics warrant such as a more favorable demand for iron oxide, aeration is used to precipitate the iron as ferric and ferrous oxide. Aeration further reduces the need for substantial pH changes and corresponding chemical input needs. The formed iron oxide is readily separable from the electrolyte.

The resulting filtrate is then ready for readjustment to the preferred electrolyte prior to returning it to the electrolysis. The electrolyte utilized in the electrolysis preferably contains at least 1 molar amount of halogen up to the saturation point of the halide at the electrolysis temperatures. The halide ion is preferably added as the alkali metal salt, alkaline earth metal salt, the halide acid, halide gas or mixtures thereof. Thus, the molar concentration referred to is that of the halogen itself. While all of the various halides, including chloride, bromide, fluoride and iodide, can be used, as a practical matter only the chloride is normally economically feasible. Of the chloride salts, sodium chloride is the most preferred, although lithium chloride, potassium chloride, calcium chloride, magnesium chloride, barium chloride and the like can be used with correspondingly good results. The particular choice of halide salts rests largely on economical considerations and the availability of the salt at the particular mining site.

In addition to utilizing a halide salt to achieve the desired hydrogen ion concentration in the electrolyte, the corresponding halide acid can be used to increase the halide ion concentration while acidifying the electrolyte to the desired pH. Thus, utilizing chloride as the halogen, hydrochloric acid is preferably used to adjust the pH to the desired level. Also, as noted previously, when utilizing sodium chloride as the halide, sodium hydroxide is preferably used in the hydroxide addition.

The concentration of halogen ion in the electrolyte requires at least a 1 molar concentration up to the saturation point of the halide. Such concentrations are often measured in terms of Baume (Be), which means that the electrolytes of the present process are at least 6 Be. it is preferred that the electrolyte be at least 10 Be with the most preferred electrolytes being in the range of to 23 Be. With sodium chloride, this represents a solution having a preferred molar concentration of 2 to 4 moles, it being recognized that the addition of acid, such as hydrochloric acid, will increase the Be, particularly with the lower salt concentrations.

The strong acid used to acidify the electrolyte is preferably hydrochloric acid but can also be any other available strong halogen containing acid or sulfuric acid, phosphoric acid and the like.

The acidity of the electrolyte is adjusted to reduce it to below a pH of about 5. The acidity can range down to a pH of less than i, i.e. as low as about a pH of 0.01. Whiie such low pl-ls are useful as electrolyte fed to the electrolysis area where ground water leakage tends to dilute the acidity, the more preferred pH electrolysis operating range is about 1.5 to 3.5.

The temperature of the electrolyte solution during the metal and sulfate separation treatment is not critical. Such temperatures can conveniently be the ambient temperatures which would normally be in the range of 10 to lOO degrees centigrade. Withdrawn electrolyte is normally at a temperature near that at which the electrolysis is carried out which often is preferably in the range of 50 to 100 degrees centigrade. However, due to cooling effects during various seasons of the year, lower temperatures are often encountered but such temperatures do not greatly affect the operation other than the effect on solubility of ions being removed. Thus, with varying temperatures some adjustment of residency times and other conditions might be warranted. However, since the reactions involved are inorganic ionic reactions which are substantially instantaneous, the temperatures within the noted ranges do not greatly affect the reaction rates. Under certain processing conditions, however, it may be desirable to reduce the temperature of the electrolyte to reduce the solubility of the salts being precipitated.

While there have been described more particularly various preferred embodiments of the present invention, it will be readily recognized by those skilled in the art that various modifications herein can be made without departing from the spirit of the invention. As such, they are intended to cover the invention broadly, being limited only by the following claims.

What is claimed is:

1. A method for recovering metal and sulfate values from electrolyte solutions withdrawn from the in situ electrolysis of sulfide ores wherein said solutions are acidic and contain copper, iron, sulfate and halogen ions comprising separating the solids from said withdrawn electrolyte, adding a calcium, barium or strontium alkaline earth metal oxide to said electrolyte to thereby form an insoluble precipitate with said sulfate ions, separating the precipitate from said electrolyte, precipitating iron from said electrolyte by pH increases and/or aeration, separating said iron precipitate and readjusting said electrolyte by the addition of a strong inorganic acid and a halogen to an acidic pH and a halogen ion concentration of at least 1 molar returning the electrolyte to the cell and continuing the electrolysis.

2. The method of claim 1 wherein the iron is removed from said electrolyte by the addition of hydroxyl ions to increase the pH to above about 10 and to thereby precipitate ferric and ferrous hydroxide.

3. The method of claim 2 wherein the hydroxyl ions are added in the form of alkali metal, alkaline earth metal or ammonium hydroxide.

4. The method of claim 1 wherein the iron is removed from said electrolyte by the aeration of the electrolyte to thereby precipitate commercial iron oxides.

5. The method of claim 1 wherein the steps are carried out at an electrolyte temperature in the range of 10 to degrees centigrade.

6. The method of claim 1 wherein copper ions are separated from said electrolyte prior to increasing the pH thereof by passing the solution over scrap iron.

7. The method of claim 1 wherein the alkaline earth metal oxide added is lime.

8. The method of claim 1 wherein the strong inorganic acid is selected from the group consisting of hydrochloric acid, sulfuric acid and phosphoric acid.

9. The method of claim 1 wherein the halogen salt is an alkali metal salt of chlorine.

10. The method of claim 9 wherein the strong acid added is hydrochloric acid and the alkali metal salt is sodium chloride. 

1. A METHOD FOR RECOVING METAL AND SULFATE VALUES FROM ELECTROLYTE SOLUTIONS WITHDRAWN FROM THE SITU ELECTROLYSIS OF SULFIDE ORES WHEREIN SAID SOLUTIONS ARE ACIDIC AND CONTAIN COPPER, ION, SULFATE AND HALOGEN IONS COMPRISING SEPARATING THE SOLIDS FROM SAID WITHDRAWN ELECTROLYTE, ADDING A CALCIUM, BARIUM OR STRONTIUM ALKALINE EARTH METAL OXIDE TO SAID ELECTROLYTE TO THEREBY FORM AN INSOLUBLE PRECIPATE WITH SAID SULFATE IONS, SEPARATING THE PRECIPITATE FROM SAID ELECTROLYTE, PRECIPITATING ION FROM SAID ELECTROLYTE BY PH INCREASES AND/OR AERATION, SEPARATING SAID ION PRECIPITATE AND READJUSTING SAID ELECTROLYTE BY THE ADDITION OF A STRONG INORGANIC ACID AND A HALOGEN TO AN ACIDIC PH AND A HALOGEN ION CONCENTRATION OF AT
 2. The method of claim 1 wherein the iron is removed from said electrolyte by the addition of hydroxyl ions to increase the pH to above about 10 and to thereby precipitate ferric and ferrous hydroxide.
 3. The method of claim 2 wherein the hydroxyl ions are added in the form of alkali metal, alkaline earth metal or ammonium hydroxide.
 4. The method of claim 1 wherein the iron is removed from said electrolyte by the aeration of the electrolyte to thereby precipitate commercial iron oxides.
 5. The method of claim 1 wherein the steps are carried out at an electrolyte temperature in the range of 10 to 100 degrees centigrade.
 6. The method of claim 1 wherein copper ions are separated from said electrolyte prior to increasing the pH thereof by passing the solution over scrap iron.
 7. The method of claim 1 wherein the alkaline earth metal oxide added is lime.
 8. The method of claim 1 wherein the strong inorganic acid is selected from the group consisting of hydrochloric acid, sulfuric acid and phosphoric acid.
 9. The method of claim 1 wherein the halogen salt is an alkali metal salt of chlorine.
 10. The method of claim 9 wherein the strong acid added is hydrochloric acid and the alkali metal salt is sodium chloride. 